Power over Ethernet (PoE), which is outlined in IEEE standard 802.3™-2005 clause 33 (the PoE standard), refers to a technique for delivering power and data to an electronic device via a network cable, such as a twisted pair Ethernet cable. In a PoE system, power sourcing equipment (PSE) provides power via an Ethernet cable to electronic devices, which may be referred to as powered devices. PoE eliminates the need for a separate power source to deliver power to attached powered devices (PDs). Such powered devices may include Voice over Internet Protocol (VoIP) telephones, wireless routers, security devices, field devices to monitor process control parameters, data processors, and other electronic devices.
The PoE standard specifies a process for detection of a PoE powered device that is coupled to the PSE before supplying power via the Ethernet cable. To perform the detection process, the PSE provides a voltage level that is within a range of 2 volts to 10 volts DC on pairs of wires of the Ethernet cable and monitors a return current (micro-amps) or applies a small current to the wires and monitors a return voltage (V) to detect a resistance signature of the device within an expected range (approximately 25 K-ohms) between the twisted pair wires. If the PSE does not detect a valid resistance, the PSE does not apply power to the Ethernet port assigned to the device.
When two PSE devices are attached together, and one of these PSE devices is unpowered, the actively powered PSE may apply power to the unpowered PSE (uPSE). A PSE applying power to an uPSE may lead to issues when the uPSE is subsequently powered. The uPSE may detect power across the output and conclude that a field effect transistor (FET) has shorted, thus leading to fault detection and error reporting.